Solenoid operated directional valves that have hydraulic cushioning means associated with the spool thereof are known in the art. Thus, it is conventional practice to provide a solenoid operated directional valve having a valve body provided with passageways for directing flow of hydraulic fluid, a spool mounted for movement in the valve body and movable to selected positions for controlling flow of hydraulic fluid through the passageways, oppositely acting solenoids arranged to actuate the spool in one direction or the other, cushioning chambers in the valve body adjacent to each solenoid for containing hydraulic fluid and into which the ends of said spool can enter, the ends of said spool defining attenuation elements for displacing fluid in the chambers so as to cushion the movement of the spool, and said valve body having a transfer loop providing communication between said chambers for restricted displacement of the fluid from one chamber to the other in response to movement of the attenuation elements.
Directional valves of the foregoing character normally include bleed ports and passageways for removal of air from the cushioning chambers and the transfer loop at initial start-up or following repair or maintenance of the directional valve. Allowing air to remain in the cushioning chambers and transfer loop can be very detrimental to the directional valve and will produce hammering and will cause the valve to malfunction.
To further complicate matters, removal of air from the cushioning system must be done at the site of installation of the directional valve, often by personnel who fail to perform the operation properly. Still further, many directional valves have the outlet of the bleed passageways at the upper side of the valve, located under the electrical junction box mounted thereon, which situation increases the time and work required for preparation to carry out the air purging operation. The time factor for then carrying out the purging operation is also unfavorable with respect to the prior art units. Very commonly, as many as several hundred cycles of operation of the prior art directional valve is required before the cushioning system is satisfactorily filled with hydraulic fluid to enable the valve to function properly.
Thus, there is a significant need for improvements in the art to enable the air to be purged quickly and effectively and in a manner that is automatically carried out without the need to disassemble the directional valve or to have the purging operation performed by inexperienced or unskilled operators.